Many of the presently available solar heating systems include collector troughs for holding water in turn heated by solar radiation. The water is supplied from appropriate water tanks and after heating is returned to the water tanks, the tanks themselves serving as a storage for the heated water.
While such solar water heating systems have been successful, in the event there is a succession of cloudy days, the water in the storate tanks can become sufficiently cooled off that it is no longer effective as a heating medium or simply as use as hot water. It would be desirable if there were some type of supplemental heating arrangement so that the water could be maintained at a desired heated level even though solar energy might not be available. A desirable feature of any such supplemental heating means would be its rapid response to any drop in water temperature below a given set minimum amount.